1. Field of the Invention
The present invention relates to an image forming apparatus and a sheet conveyor and more particularly to a sheet conveyor, when transferring a developer image onto a sheet, regardless of a thickness of the sheet, for preventing a displacement of the developer image, an image forming apparatus having the sheet conveyor, and a sheet conveying method.
2. Description of the Related Art
In an image forming apparatus such as a color copy machine or a printer, an image forming apparatus of a type of arranging a plurality of image forming units along a photoconductor in a belt shape or sheet shape, superimposing developer images formed by the respective image forming units on the photoconductor in the belt shape or sheet shape, transferring them onto a sheet in a batch, thereby obtaining a color image is known.
In an image forming apparatus for transferring developer images onto a sheet, thereby obtaining image output, a method for stopping once the sheet guided toward the image transfer position before the image transfer position and aligning the sheet position to the developer image position is used widely. Further, it is known that an error in the stop position when the sheet is stopped once before the image transfer position causes a displacement of the developer image (from the sheet) transferred to the sheet, that is, a variation in the size of the marginal portion.
Therefore, as disclosed in Japanese Patent Application Publication No. 2000-95394, it is proposed to install a brake clutch in at least one of a pair of rollers called an aligning roller pair for temporarily stopping a sheet and prevent a shock at start time of rotation of the aligning roller pair.
On the other hand, it is known that when a sheet is thicker than a fixed thickness and the rear end of the sheet is separated from the rollers of the aligning roller pair (gets out of the position pressurized between the rollers), the respective rollers are applied with force by which the rollers intend to closely contact to each other by the pressure applied to the rollers, thereby are pressed out (the speed for conveying the sheet is increased temporarily). This varies the speed of the sheet being conveyed at the image transfer position, thus the transferred developer image is displaced.
Further, by the art disclosed in Japanese Patent Application Publication No. 2000-95394 aforementioned, the aforementioned temporary increase in the conveying speed caused when the sheet is thicker than the fixed thickness and the displacement of the transferred developer image due to a change in the speed of the sheet conveyed at the image transfer position are not canceled.